Scientists ponder the question, "What advances
in power technology are required to send human and robotic explorers
throughout the solar system?"

September
3, 2002: Beyond all the planets in our solar system in a
cold, dark, empty region of space, Voyager 1 continues its 25-year
journey of exploration. It's headed for the heliopause, that
boundary where the Sun's influence ends and the dark recesses
of interstellar space begin. From where Voyager sits, the Sun
is merely the brightest star in the sky--seven thousand times
dimmer than we see it from Earth.

Voyager doesn't have any solar panels; they wouldn't do any
good so far from the Sun. The probe stays in touch by carrying
its own power source, an early radioisotope thermoelectric generator
(RTG), which converts the heat generated from the natural decay
of its radioactive fuel into electricity. Its RTG will supply
Voyager with electricity at least until 2020.

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Space probes that travel much beyond Mars need more power
than solar cells can provide. Another example is the Ulysses
spacecraft. It was launched in October 1990 from the space shuttle
on a mission to study the Sun's poles. To get above the Sun,
Ulysses had to fly around Jupiter and slingshot out of the plane
of the planets. Near Jupiter, the Sun's rays are 25 times weaker
than near Earth. Solar panels large enough to catch this weak
energy would have weighed 1,200 pounds, doubling the weight of
the spacecraft and making it too heavy for booster rockets from
the shuttle. Instead, Ulysses was equipped with an RTG weighing
only 124 pounds. It easily powers all the probe's onboard systems,
including navigation, communication and scientific instruments.

A probe like Ulysses needs a couple hundred watts of power
to operate onboard systems. For comparison, the shuttle's onboard
systems use 5 to 10 kilowatts (kW) of power, 50 times that. The
International Space Station (ISS) uses 10 times more, or about
100 kW for onboard systems.

The ISS never leaves Earth orbit, which reduces the power
it needs. Human missions beyond Earth's neighborhood, however,
will require power not only for onboard systems, but also for
propulsion and for systems to support humans when they arrive
wherever they're going. "To pursue ambitious human missions
across the solar system, perhaps returning to the Moon, perhaps
going on to Mars, will require hundreds to a thousand kilowatts
on the surface and hundreds to thousands of kilowatts for transportation
systems," says John Mankins, chief technologist for the
Advance Systems Program at NASA headquarters. You can't just
plug into the nearest electrical outlet, he added. You have to
bring your own power source. Ideally, you'd like to find something
that could provide power for both propulsion and operations.

Below: Chemical rockets propel the space shuttle away
from Earth.

Since
Robert Goddard's first test launch of a rocket in 1916, space
missions have used chemicals to get the acceleration needed to
escape Earth's gravity. A rocket's 5- to 15-minute burn sends
the spacecraft towards its destination; then it coasts the rest
of the way unless it uses the gravity of other planets for an
additional boost. For Voyager, it took years to reach Saturn
and then the spacecraft was only able to spend days in the Saturn
system and only hours near the planet itself.

Mission planners would like to do better in the future.

From the perspective of the Exploration Office at the Johnson
Space Center, Jeff George sees "an evolving family of related
power and propulsion technologies" for the next wave of
human exploration. The first likely candidate is electric propulsion
(EP). You don't need as much thrust in space as you do to escape
Earth's gravity, explains George, but you do need to produce
thrust using very little fuel because of weight restrictions.
Electric propulsion could provide fuel-efficient thrust after
an initial chemical boost into space.

Specific impulse--that is, the pounds of thrust produced per
pound of propellant used per second--is a measure of the efficiency
with which a system uses fuel to produce thrust. Higher is better.
The space shuttle, which stays near Earth, uses chemical propulsion
with a specific impulse of 450 seconds or 450 pounds of thrust
for a pound of propellant per second. EP has 10 times the specific
impulse of chemical propulsion and potentially can go as high
as 10,000 seconds.

EP
got its first try in 1998 on Deep Space 1--a spacecraft that
tested many new technologies before it flew by comet Borrelly
in 2001. Deep Space 1 needed 2.5 kW to power both its electric
ion propulsion drive (pictured left) and other onboard systems.
The energy came from an innovative collector consisting of advanced
solar cells and a lens to concentrate sunlight on the panels.
Together they achieved a 23% efficiency in converting sunlight
to electricity compared with 14% efficiency for the solar arrays
on the ISS.

Above: The ghostly blue exhaust
of Deep Space 1's ion propulsion engine. Power collected from
the craft's solar arrays is used to ionize atoms of xenon. As
these ions are expelled by a strong electric field out the back,
the spacecraft slowly gains speed.

Building on the success of Deep Space 1, a new mission named
"Dawn" will leave Earth in 2006. Propelled by an ion
engine with a specific impulse of 3100 seconds, Dawn will travel
to Ceres and Vesta, two of the biggest asteroids in the solar
system. Although Ceres and Vesta lie farther from the Sun than
Mars does, the spacecraft will be able to draw all the power
it needs from 7.5 kW solar arrays.

Manned missions need more power. "The next step for a
[human-crewed] Mars mission," says Jeff George, "is
to step up to 5-10 megawatts of nuclear power and then scale
up the electric thrusters to megawatts per engine." Going
from kilowatts to megawatts is not a simple problem. NASA is
now working on a 5-10 kW next-generation ion propulsion system.
George envisions small, nuclear-electric vehicles of 100-200
kW exploring the outer planets as a pilot version of the megawatt
scale they'd like to use for human exploration.

Above: Fission, the same atom-splitting process that
energizes modern nuclear power plants, is one way to generate
high levels of power to propel spaceships.

To run a megawatt EP system, you need a source with both high
energy and high power. As John Cole, manager of the Revolutionary
Propulsion Research Project Office explained, "Energy is
the most important factor, but power (the energy released per
unit time) determines acceleration." So what source provides
enough power? "Nuclear has plenty of energy--and potentially
plenty of power, too," Cole observes. "Solar panels
provide insufficient power for the entire vehicle to accelerate
to levels that permit short trip times."

Radioisotope
power sources (like the RTGs onboard Voyager) give off a lot
of energy over a long period of time, but not a lot of power,
only tens to hundreds of watts. To get kilowatts to megawatts
of power, you have to go to nuclear fission, says Les Johnson,
of NASA's Advanced Space Transportation Program.

Right: Radioactive decay, pictured here, is the energy
source for RTGs. It's not as potent as nuclear fission.

Fission, in which a neutron splits an atom into two radioactive
isotopes, is the process nuclear power plants on Earth use to
produce electricity. "Bringing along a fission reactor on
a spacecraft would be like bringing along your own [mini] power
plant," says Johnson. A fission reactor is capable of fueling
high-performance electric propulsion beyond the inner solar system.
It is longer duration and power rich for performing sophisticated
scientific investigations, high-data rate communications, and
complex spacecraft operations.

That's a pretty good resume for fission, but it still doesn't
pass John Cole's test. Cole set himself the requirement of getting
humans to the outer planets in a year and back in a year. Nuclear
fission has enough energy, but not enough power to provide the
acceleration needed. NASA is designing a 300-kW flight configuration
system using nuclear fission. But to meet Cole's test, "one
needs a very high specific power, power per unit mass vehicle
three orders of magnitude better than what we've currently planned
for nuclear fission." For that, you have to step up to nuclear
fusion--the same process that powers the Sun and stars.

Above: Go out at night and look at the sky. Every star
you see is a fusion reactor. Scientists would like to harness
such power to propel spaceships and energize distant colonies.
[more]

Fusion, which releases energy by combining rather than splitting
atoms, could in principle supply gigawatts of clean power. However,
fusion propulsion systems as we understand them today would be
very big, requiring a vehicle the size of the space station or
Battlestar Galactica, weighing hundreds of tons--although the
size might come down with research.

Fusion engines would be very efficient fuel burners with a
specific impulse of 100,000 seconds. "Though
we couldn't do it in 10 years, if we could launch a fusion propulsion
system 10 years from now, we could send a vehicle out to catch
Voyager and bring it back," says Cole. That kind of power
and speed shortens the time that astronauts would be exposed
to harmful cosmic radiation and the bone loss that comes from
prolonged weightlessness.

Perhaps there's something even better than fusion: A thruster
powered by matter-antimatter annihilation would have a specific
impulse of 2,000,000 seconds, according to Cole.

It
sounds like science fiction, but researchers are learning to
create and store small amounts of antimatter in real-life labs.
A portable electromagnetic antimatter trap at Penn State University,
for example, can hold 10 billion
antiprotons. If we
could learn how to use such antimatter safely, we could impinge
some on a thin stream of hydrogen gas to create thrust. Alternatively,
a little antimatter could be injected into a fusion reactor to
lower the temperatures needed to trigger a fusion reaction.

Right: This "Penning trap" developed at Penn
State University stores antiprotons. [more]

"Propulsion isn't the only reason to go nuclear,"
notes Colleen Hartman, director of solar-system exploration at
NASA headquarters. "Onboard systems benefit, too. The excess
power is like getting the Las Vegas strip instead of a single
light bulb. It gives you greater communication and mission flexibility."

The Mars Smart Lander and Mobile Laboratory, slated for launch
as early as 2009, was originally conceived as a solar-powered
mission. But now researchers are considering an upgrade from
solar to nuclear power: "Putting nuclear power on board
will extend the mission from 3-6 months [with solar power] to
5 years [with radioisotope power]," says Ed Weiler, head
of the Space Science Enterprise at NASA headquarters. "It
will enable the rover to drive to a location rather than having
to land there. The bandwidth for data communication goes way
up, and the rover can work 24 hours a day. Everything increases
by a factor of 10 when you add an RTG to a mission."

Scaling up from the Mars Lander to a human mission on Mars
requires more power--about 30 kW to heat and cool a human habitat,
run computers and lights, make oxygen, recycle water and recharge
the rovers, says Jeff George. For a long mission "we don't
have the kind of energetics where you can dash back home [in
case of trouble]," adds Gary Martin, assistant associate
administrator for Advanced Systems in NASA's Office of Space
Flight. "You're building things that have to be ultra reliable,
self-healing, and autonomously sense when they're hurt."
Broken parts will have to be made or repaired on site: you can't
bring spare parts. Power-intensive processes like making parts
or producing propellant for leaving Mars would be another 60
kW, according to George.

In the end, one power source does not fit all needs. Looking
at the big picture, John Mankins says "we need very high-efficiency,
high-power electric propulsion for interplanetary travel; we
need reliable and affordable high-energy chemical propulsion
systems for going up and down from planetary surfaces; and we
need to be able to store chemical or solar power in order to
live and work on the surface. Robots could use radioisotope power;
and there's reactor power and wireless beaming to consider as
well."

The choices are many, yet one thing is clear: Wherever we
go in space and whatever we do there, we'll need more power.

Right:
On January 16, 1959, this photograph appeared in a Washington
DC newspaper. The headline proclaimed "President Shows Atom
Generator." President Eisenhower and a group of officials
from the Atomic Energy Commission (AEC) officials in the Oval
Office of the WHite House. There were gathered around the president's
desk staring at an early-model RTG, dubbed the world's first
atomic battery. [more]

Nuclear power in space -- a thorough discussion from the University
of Missouri. Pictures include Eisenhower with the prototype RTG,
an RTG being replaced by an astronaut on the Moon, and the trajectory
of the Ulysses spacecraft.